ORIGIN OF DETRITAL FLUXES IN THE SOUTHEAST INDIAN-OCEAN DURING THE LAST CLIMATIC CYCLES

Because of a close relationship between detrital flux variations and m agnetic susceptibility (MS) flux (MS cm(-3) of bulk sediment multiplie d by the linear sedimentation rate) variations in the southeast Indian basin of the southern ocean, MS flux profiles have been used to exami ne the spatial and temporal detrital flux changes in this basin during the last climatic cycle. Results indicate a general increase in detri tal material input during the coldest periods, suggesting a widespread phenomenon, at least on the basin scale. Mineralogical data, geochemi cal data, and Sr-87/Sr-86 isotopic ratios have been used to determine the origin and transport mechanisms responsible for increased detrital flux during glacial periods. Mineralogical and geochemical data show that these glacial ''highs'' are due to increases in both Kerguelen-Cr ozet volcanic and Antarctic detrital inputs. The Sr-87/Sr-86 isotopic composition of the >45-mu m fraction indicates that the Kerguelen-Croz et province contributes to at least 50% of the coarse particule input to the west. This contribution decreases eastward to reach less than 1 0%. These tracers clearly indicate that the Crozet-Kerguelen province was a major source region of detrital in the western part of the basin during glacial times. In contrast, material of Antarctic origin is we ll represented in the whole basin (fine and coarse fractions). Because of the minor amount of coarse particles in the sediments, volcanic pa rticles from Kerguelen and crustal particles from Antarctica have most probably been transported by the Antarctic bottom water current and/o r the Circumpolar deepwater current during glacial periods as is the c ase today. Nevertheless, the presence of coarse particles even in low amount suggests also a transport by ice rafting (sea-ice and icebergs) , originated from both Kerguelen and Antarctic sources. However, the r elative importance of both hydrographic and ice-rafting modes of trans port cannot be identified accurately with our data. During low sea lev el stands (glacial maximum periods), increasing instability and erosio n of the continental platform and shallow plateaus could have resulted in a more efficient transfer of crustal and volcano-detrital material to the Southeast Indian basin. At the same time, extension of the gro unded ice shelves over the continental margins and increase in the ero sion rate of the Antarctic ice sheet could have induced a greater inpu t of ice rafted detritus (IRD) to southern ocean basins. Enhancement o f the circumpolar deepwater current strength might have also carried a more important flux of detrital material from Kerguelen. However, an increase in the bottom water flow is not necessarily required.